Add el-geometry

[mahdiehmalekian]

Introduces a new package tree for clique embedder and adds the first module, el_geometry.py, which implements the pure geometry of the L-shaped ("el") templates used to build clique embeddings on a defected Zephyr graph.

Changes
minorminer/embedders/__init__.py
minorminer/embedders/clique/__init__.py
minorminer/embedders/clique/_zephyr/__init__.py
New module minorminer/embedders/clique/_zephyr/el_geometry.py computes ideal el-template geometry for Zephyr. All functions are k-agnostic (quotient) and depend only on the Zephyr grid size m and the ideal topology, not on the tile size t or on which nodes/edges a specific graph contains. Uses Zephyr Cartesian (x, y, k) coordinates following dwave.graphs.ZephyrCartesianCoord.

Matching tests:
tests/embedders/__init__.py, tests/embedders/clique/__init__.py, tests/embedders/clique/zephyr/__init__.py,
tests/embedders/clique/zephyr/test_el_geometry.py

Note
This is the first piece of a larger Zephyr clique-embedder effort; it lands the geometry layer only, with no public API wired up yet.

AI use
Used AI to clean up my code and complete, edit documentation. Also used it to write the test suite. Checked and edited everything myself.

[mahdiehmalekian]

While this module works with the concept of Zephyr Cartesian coordinates introduced in dwavesystems/dwave-graphs#275, this module is independent of that PR.

[mahdiehmalekian]

Note to reviewers: This is the first of a series. Subsequent clique-embedder PRs depend on this PR and can only follow once this merges. So would much appreciate a speedy review.

[mahdiehmalekian]

Undid the creation of embedders directory and moved el_geometry.py from embedders/clique/_zephyr to utils/_clique per Slack discussion